Forget about it, nearly everything is different: box, engine, ECU, ABS pump, engine wiring loom, dashboard wiring loom, etc... You better off spending the 2-3 grand towards the car with the engine you want in it or spend the money upgrading the car you have...
3k spent into a 2,5 tdi will go beyond what a 3.0Tdi will do with just a remap ! Done a DPF delete on a 2007 a5 2.7Tdi this weekend and working on the new engines is much more involved compared to the old v6 tdi lump !